This invention relates to packaging of an optoelectronic element and, more particularly to such packaging for an optical-electronic interface module having a multipin housing made of conductive plastic. A rigid double-sided printed circuit board (PCB) enclosed in the housing couples the optoelectronic element to pins protruding from the housing.
Fiber optic technology is widely used in conjunction with electronic circuits for a variety of applications. It is most favorable in the field of communications since fiber-optic transmissions are less susceptible to radio-frequency interference (RFI) and electromagnetic interference (EMI) that is not of a sinusoidal character as compared to electronic transmissions, and data rate for digital transmission can be much higher through optical fibers.
The coupling between optical fibers and electronic circuits has posed numerous problems. Due to the relatively low amount of power used in transmission and reception of signals through optoelectronic elements, the leads to and from the device are susceptible to EMI, which may so mask the electronic signal as to make the information received and/or transmitted unreliable. Also, long leads limit the maximum bit rate of both transmission and reception due to parasitic inductance and capacitance of the leads. Therefore it is necessary to have the optoelectronic element and the receiving and/or transmitting electronic circuit as close to each other as possible with effective shielding against EMI. Optoelectronic devices and interfacing electronic circuits are therefore preferably housed in the same package.
Many different packages have been designed to solve the common problems discussed above. U.S. Pat. No. 4,647,148 discloses a prior-art fiber optic receiver in a metal housing 12 that slips into a metal case 19. Such a package provided effective EMI shielding but presented other problems, namely parasitic inductance and capacitance of input and output leads. In addition to the problem of such parasitic impedance, there was a problem of sealing circuit components against moisture. Moreover, precision fabrication required of the metal housing 12, case 19 and receptacle 17 in order to have proper mating of various parts added to the costs of packaging. To overcome the parasitic impedance problem, the prior-art package having long input leads and four output leads described in that patent was superseded by a dual-in-line multipin package with a one-sided PCB. To then provide shielding for all elements from EMI, the PCB was formed on a ceramic substrate with a seal ring 22 mounted on a metal plate 29. A conductive shell seam welded onto the seal ring 22 completed what is equivalent to the prior-art housing 12. A metal cover 34 was then placed over the entire assembly comprising the PCB with a seal ring on a metal plate, a conductive shell over the seal ring, and a receptacle 28 with an optoelectronic device 20 in a sealed can 27. This provided airtight sealing for the PCB coupling.
U.S. Pat. No. 4,911,519 discloses a plastic frame 52 for a dual-in-line integrated circuit package 66 and an optical transmitter and/or receiver assembly 60. To provide EMI shielding, a metal plate 75 and metal cover 76 were used to protect the optoelectronic element. That packaging relied upon a sealed integrated circuit (IC) package and was therefore limited to the IC function that can be provided within the sealed IC package.
U.S. Pat. No. 4,979,787 discloses a nonconductive plastic housing for an optical-electronic interface module with a folded printed circuit board to increase the circuit board area on the one side that is folded in to accommodate more integrated circuits for more signal conditioning functions and a metal film on the outer surface of the folded PCB for EMI shielding. This module was an improvement over the prior art in that it allowed more circuit functions to be housed close to the optoelectronic element without increasing the footprint of the housing, although still having some problems, such as failure in the connections from one part of the folded circuit board to another as a result of folding the circuit board and placing it in the plastic housing after fabrication. Therefore, the need still exists for a well shielded, cost effective, miniature package for housing optoelectronic elements with more complex interfacing integrated circuit functions.